1. Field of the Invention
The present invention relates to oil-external emulsions and, in particular, to the thinning of emulsions of the water-in-oil type in which an emulsifying agent comprising a fatty acid or an amine or an amide derivative of a fatty acid is utilized.
2. Description of the Prior Art
Oil-water emulsion type drilling fluids have been used advantageously in the oil well drilling industry for many years. Such emulsion drilling fluids possess many advantages over regular muds such as bringing about increased drilling rates, longer bit lives, improved hole conditions and the like. The most commonly used emulsion drilling fluids are oil-in-water types wherein oil is the dispersed phase and water the continuous phase. Inverted or water-in-oil emulsions wherein oil is the continuous phase and water is the dispersed phase also have been used to advantage.
The present invention relates to drilling fluids comprising such water-in-oil emulsions. Such drilling fluids require an emulsifying agent which functions to maintain the dispersion of the water in the oil and provide the proper rheological properties to the fluid to permit its use as a drilling fluid. The manner of utilizing drilling fluids and, in particular, those of the emulsion type is well known in the art and, therefore, hereafter only will be described in general terms.
In rotary drilling operations, a drilling fluid is pumped down the drill stem of the rotary rig and out through openings in the drill bit attached to the drill stem. The drilling fluid flows around the drill bit and returns to the surface through an annular passage between the drill stem and the walls of the well bore. The primary purpose of the drilling fluid is to cool and lubricate the drill bit and to carry the formation cuttings created by the drill bit away from the bit and up to the earth's surface.
In order to prevent the loss of fluid into porous or permeable formations traversed by the well bore, the fluid contains a suspended or dispersed solid material such as, clay or other suitable materials which will filter out upon the walls of the well bore to form an impermeable coating through which the liquid portion of the drilling fluid cannot pass. Also, in order that the fluid may exert a pressure against the walls of the well bore to prevent the collapse thereof, most of the drilling fluids presently employed are "weighted" by having suspended therein a high density solid such as barite, calcined clay, calcium carbonate, or the like. Heretofore, it has been difficult to achieve high apparent densities with conventional weighting materials and maintain fluid pumpability when the liquid portion of the fluid took the form of a water-in-oil emulsion. This arose out of the fact that such weighting materials were for the most part oleophobic, that is, they were not readily wet by the continuous oil phase of the emulsion, and hence tended to settle out of the fluid rather than remaining substantially uniformly suspended therein.
This problem has been at least partially solved in such emulsion-type drilling fluids by utilizing emulsifying agents comprising fatty acids or amine or amide derivatives of such fatty acids. Such emulsifying agents permit the weighting materials to be dispersed in the oil phase of the fluid.
A primary requirement of a drilling fluid is that the fluid must possess the desirable rheological properties when compounded and also must be able to retain these properties, within certain limits, during continued use of the fluid and particularly under conditions of elevated temperature bottom hole conditions.
As previously indicated, the drilling fluid removes cuttings from the well bore as drilling progresses. This process is governed by the velocity at which the fluid travels up the annulus of the well bore, as well as its viscosity or flow properties and its density. The cuttings removal efficiency usually increases with increasing viscosity and density. The viscosity depends upon the concentration, quality and state of dispersion of suspended colloidal solids in the drilling fluid. Continued use of a water-in-oil emulsion type drilling fluid normally results in an increase in the viscosity of the fluid because of an accumulation of colloidal drill cuttings in the fluid. Proper control of viscosity and gel strengths is essential for efficient cleaning of the well bore, suspension of the weighting material and cuttings when circulation is interrupted and to minimize pressure losses and swab or surge pressures when moving the drill string. The term "gel strength", as used herein, means the viscosity developed by the fluid upon standing for a period of time. The viscosity of a drilling fluid can be reduced by thinning the fluid. Thinning is obtained by reducing the plastic viscosity, yield point or gel strength or a combination of these properties of the drilling fluid. Thus, viscosity can be reduced by decreasing the solids content or the number of particles per unit of volume. Typically, this is achieved by diluting the drilling fluid with a liquid such as diesel fuel or the like in water-in-oil emulsions. While this reduces the viscosity of the fluid, it also has the undesirable effects of reducing the weight of the fluid and changing the oil:water volumetric ratio. Thus, additional weighting material then must be added to the drilling fluid to provide the desired fluid weight and the oil:water ratio adjusted before it can be recirculated through the drill bit.
It would be desirable to provide a chemical which can be utilized to reduce the viscosity of a drilling fluid and which does not adversely affect the other properties of the fluid.